1. Field of the Invention
The invention relates to a process and a device for correcting the thermal ovalisation (or out-of round state) appearing on rolls for the continuous casting of metal strip.
2. Description of Related Art
A machine for the continuous casting of metal strip generally comprises two identical rolls located face to face, separated by a space having the thickness of the metal strip to be produced and rotating in opposite directions to one another.
The liquid metal is supplied from one side of the space whereas the strip leaves from the other side at its nominal thickness.
Strips ranging from a few centimetres in thickness to several millimetres or less can be produced with such a device.
FIGS. 1 (longitudinal section) and 2 (cross section) show the general structure of a roll.
It comprises at (1) a roll body (central portion) which is surrounded by a shell (2) which receives the molten metal and serves to roll the strip. It is therefore necessary to cool the assembly.
Cooling is usually carried out by a coolant (generally water) circulating in at least one cooling circuit located inside the roll body (1).
This circuit comprises at least one cold water supply hole (3) in the form of a tube pierced in the roll body (1) parallel to its axis, opening at the exterior through one of its ends, the other end being blocked, and extending in line with the shell (2) over its entire width.
A plurality of smaller diameter tubes (4) connects each supply hole (3) to a distributing manifold (5) in the form of a groove located beneath the shell (2) and substantially parallel to the supply tube (3); each manifold supplies water to an actual cooling device comprising a network of small channels (5) machined at the peripheral surface of the roll body (1) and located beneath the shell in a transverse plane; the water circulates there, makes thermal contact with the shell and cools it.
After the water has been heated it is discharged by means of a device identical to the supply device. It comprises (see bottom part of FIG. 1 and FIG. 2) a manifold (7) for the discharging of hot water and a plurality of small diameter tubes (8) connecting it to the discharge tube (9).
A roll body usually comprises two or three or four water supply circuits and as many discharge circuits interconnected by the channels (6) of the cooling device. The reciprocal arrangement of the cold water and hot water circuits is shown clearly in particular in FIG. 2 which shows the case of two circuits. The figure shows their interconnection and their alternate arrangement, offset from one another by 90.degree.. The arrows illustrate the direction of circulation of the water. In the cases of three or four circuits, the offset is 60.degree. or 45.degree..
Thus, the cold water arriving in one of the distributing manifolds (5) is then dispersed in the cooling channels (6) situated on either side of the manifold (5), is heated and is discharged through the manifolds (7) which therefore recover the water originating from two distributing manifolds (5) located on either side.
With such an arrangement of the cooling circuits, cold and hot regions appear in the shell and the roll in the vicinity of the manifolds and tubes for the supply of cold water and discharge of hot water.
This diversity in temperature which can attain 4.degree. C. causes expansion, leading to deformation of the roll known as ovalisation or out-of round state.
This out-of-round state will be manifested by cyclic irregularities in the thickness of the cast metal strip and will therefore affect the quality thereof; the thinner the cast strip, the more inconvenient this defect.
These temperature deviations should therefore be eliminated or minimized to improve the quality and evenness in the thickness of the cast strip. Furthermore, the applicants have sought a process and a device for reducing the temperature deviations in the roll, which are effective and easy to produce or carry out and are inexpensive.